I needed a small transmitter, which would allow me to transmit good, old music into my AM-only radios. So, one Saturday afternoon I got into gear, designed and built a very crude, terribly non-optimized little transmitter. It’s almost a joke expressed in electronics, full of poor design, so please don’t think that this is the best I can do! You must see it as a quick and dirty 5-hour effort, because that’s all the time the transmitter took to design, build, and test. Making this web page about it is taking much longer! I’m putting this thing on the web only because many people have asked me to do so, despite its crude design!

As well as a number of photos documenting the build, he includes a schematic (click to expand). He describes it:

As you can see, the transmitter couldn’t be much simpler: A TTL quartz oscillator provides a 1MHz square wave which is used to directly drive a transistor in full switching mode. A tank circuit turns the square wave into an approximate sine wave, and the 50 Ohm output is taken from one eleventh of the tank capacitive reactance.

The modulation part is equally bare-bones-simple: Two input jacks accommodate stereo signals, which are simply added to form the mono signal needed to modulate the transmitter. A trimpot allows to adjust the modulation level. Setting it to the middle of its range will provide correct modulation depth with a typical line level signal, as provided by most CD players. The audio adder drives a power transistor, which modulates the supply voltage to the RF transistor. That’s pretty much all there is to it… Add a standard regulated 12V power supply, and an additional 6V regulator to bias the modulator and to power the TTL oscillator, and that’s the whole circuit.

He finishes with a small hint:

Before you consider copying this transmitter, make sure you don’t have a local AM station transmitting at or near 1.000MHz! If there is one, you would have to find a quartz oscillator for a different frequency in the broadcast band, and that might be a lot harder than finding the 1MHz oscillator! In that case, it might be better to consider using a totally different drive source.

3 Comments

For a quick and dirty transmitter, perhaps for the antique radio enthusiast, this looks as if it would do the job. Operation of low power AM transmitters like this are governed by FCC regulations with the commonly cited regulations being Part 15.209 (field strength) and 15.219 (power input to final RF amplifier and restriction of antenna and ground length). The antenna looks longer than allowed under 15.219 but perhaps the actual field strength falls equal or below that required under 15.209.

Chris

November 22, 2013 03:37

Is this legal? I don’t think so! There are rules governing radio transmitters, and as a licensed Radio Amateur I object to people building illegal transmitters, especially ones that work within the broadcast bands, that cause trouble and get us all a bad name.
I am even more surprised that you have seen fit to praise such illegal activities, let alone publish it all over the www in this manner. Does this make you an accessory?
Regards, Chris

Is this illegal for home use? And don’t in-car radio transmitters for iPhones and iPods and the like work on the same principle? Things like the iTrip were banned in the UK but only until a few years ago, IIRC. And, to be fair to homo ludens electronicus, the (US) blogger does caution against irresponsible use:
“Before you consider copying this transmitter, make sure you don’t have a local AM station transmitting at or near 1.000MHz! If there is one, you would have to find a quartz oscillator for a different frequency in the broadcast band, and that might be a lot harder than finding the 1MHz oscillator! In that case, it might be better to consider using a totally different drive source.”